Seperate by loose parts
If you are having troubles with doing this in one operation you may have better luck by breaking it into smaller pieces.


In edit mode goto side view and Box select one or two levels. Press B to start box select mode. 
Press CtrlL to select vertices linked to what is selected, this ensures you keep each piece together.
Press P and choose Selection to separate the selected portion into a new object.
Repeat until you have separated the whole tower into smaller parts.
Select one of these smaller objects and go into edit mode.
Press P and choose by Loose Parts
Repeat for each part created above.